Studies continue to define the receptors and structures involved in natural killer (NK) recognition. A cDNA coded for a unique 7 kB mRNA was cloned. Based on the predicted amino acid sequence from this cDNA, anti- sense studies were performed in rat NK cells. Studies using the rat NK cell line, RNK16, have demonstrated a functional relationship against both tumor and pathogen modified targets. Highly purified T cells were used as effector cells for cellular cytotoxicity. When either the unstimulated cells or the activated T cells had R24 (at the level of 10 micrograms/ml) added into the assay mixture, significant increases in basal cytotoxicity were observed. One possible interpretation of these data is that homotypic aggregation between distinct IgG3 molecules promoted bridging between the effector and the target cell. To test this hypothesis, the combination of R24 binding to the T cell and NR-CO4 binding to the tumor cell resulted in significant killing of LS-180 tumor cells. The murine R24 IgG3 monoclonal antibody cannot be replaced by the chimeric gamma1 version of the R24 (which lacks the cooperativity), or can the NR-CO4 be replaced by a monoclonal antibody of the non-IgG3 isotype (D612, IgG2a) directed against the colon target. The use of two IgG3 antibodies allows the initial delivery of one monoclonal antibody, permitting localization of this antibody, followed by the administration of the second antibody and results in cross-linking; and 2) the cost and difficulty of production and purification would be considerably less than bifunctional or heteroconjugated antibodies.